Remote or automatic reading of the natural gas meter usually involves the accumulation of electrical pulses produced by a device placed on the gas meter that translates mechanical motion, usually rotation of a shaft, into electrical pulses. Each pulse represents a unit volume of gas. Reliable accumulation of gas volume usage demands reliable generation of electrical pulses by the device placed on the gas meter.
As stated, current technology predominately uses a magnet mounted on a shaft which, when moved or rotated, activates a magnetically sensitive reed relay which opens and closes an electrical circuit. The opening and closing of the circuit represents the pulse conditions that are accumulated by the remote or automatic reading equipment.
The switch action of a reed switch requires no electrical power to operate and only minimum electrical power to sense the operation thereof. This minimizes the electrical power requirements of the remote or automatic reading device and is very desirable since the remote or automatic reading equipment is often battery powered. The easy means of interface and the low power requirements of the magnet and reed switch have resulted in a proliferation of these devices implemented to provide electrical switch closure pulses which represent specific gas volume amounts that are subsequently accumulated by remote or automatic reading equipment.
Increasing natural gas prices and other demand for increased gas volume measurement accuracy have motivated a desire for smaller volumes of gas to be represented per pulse. This results in higher pulse repetition rates for the reed switch devices. Operating at higher repetition rates has reduced the operating lifetime of the reed switch components and reduced reliability of systems using the current reed switch technology.
The present invention overcomes the disadvantages of the prior art by using a magnetic sensor that generates an output signal without requiring electrical power. It is of a type disclosed in U.S. Pat. No. 3,820,090. The magnetic sensor is located close to a magnet that is mounted on the shaft of the gas meter. As the shaft and the magnet rotate, magnetic flux reversals cause the sensor to produce voltage pulses that are 20 microseconds long. These pulses would not be directly compatible with the input of existing remote or automatic meter reading equipment because they are too narrow. To interface the signal with existing remote or automatic meter reading equipment, the output from the sensor must be modified. The duration of the output signal must be increased significantly and the signal must be buffered to represent an opening and closing of the external circuit that connects the pulser to the remote or automatic meter reading equipment. Thus the present invention uses a circuit that consumes no quiescent power to perform the signal modification. Finally, a MOSFET device functions as a switch element to open and close the external circuit. No source of electrical power, other than the magnetic sensor, is required by the invention to do the signal modification.
Thus several aspects of the invention are distinctive from other attempts to solve the same problem. Critical to the application is a sensor that generates a voltage signal. Other attempts have used sensors that consume electrical power rather than generate it. Still other attempts use a sensor that generates power but the signal amplitude decreases as the shaft speed decreases. Since the application requires sensing shaft speed all the way to a stop, the present invention uses a sensor with an output that is consistent for all shaft speeds useful to the application. Further other attempts at solving the problems have used active devices in the circuit to modify the sensor output signal. These active components require electrical power to operate and would be unacceptable for the application disclosed herein. One prior art device as disclosed in a document entitled "Wiegand Effect Sensors and Their Application in Manufacturing" by Joseph Pasqualucci, dated Aug. 1, 1989, discloses such a device that uses a bipolar transistor. However, the bipolar transistor has a low input impedance that would prohibit the output pulse duration required for this application.
Thus it is a feature of the present invention to provide a solid-state pulser having a separate magnetic sensor, a circuit requiring no quiescent power to increase the duration of the output signal of the sensor and a first MOSFET output transistor acting as a switch and having a high input impedance to allow the increased duration of the active output signal.
It is another feature of the present invention to provide a gas meter solid-state pulse circuit with a magnetically sensitive device and circuit that will provide easy interface to existing equipment, require no operating electrical power and operate longer than existing prior art devices.
It is still another feature of the present invention to provide a sensor that generates a voltage signal instead of consuming electrical power.
It is also a feature of the present invention to provide a sensor that senses shaft speed all the way to a stop and that has an output that is consistent for all shaft speeds useful to the automatic meter reading equipment.
It is yet another feature of the present invention to provide one output closure pulse for each changing magnetic flux cycle applied to the magnetic sensor.
It is a further feature of the present invention to have a moving magnet which applies magnetic flux of one polarity on a magnetic sensor and then applies magnetic flux of the opposite polarity on the same sensor in the course of making a complete cycle.
It is still another feature of the present invention to have the magnet rotating such that its fastest rotational motion in a particular application does not result in magnetic flux reversals on the magnetic sensor more frequent than the minimum output switch closure duration required by the particular application.
It is also a feature of the present invention to have a magnetic sensor that generates a positive voltage pulse of 20 microseconds duration when exposed to magnetic flux of one polarity and a similar negative voltage pulse when exposed to magnetic flux of the opposite polarity.
It is another feature of the present invention to conduct the positive pulse, by means of a second enhancement MOSFET, to charge a capacitor and thus provide the positive voltage to the gate of the first enhancement MOSFET, used as an output switch, so as to turn on such a device for a time longer than 50 milliseconds.
It is yet another feature of the present invention to conduct the negative pulse of the magnetic sensor, by means of a second enhancement MOSFET, to the previously charged capacitor to cause the discharge of said charged capacitor so as to apply a negative voltage to the gate of the first enhancement MOSFET, which is used as an output switch, and thus turn off such a device.
It is still a further feature of the invention to connect the gate of the second enhancement MOSFET, used to conduct the negative pulse from the magnetic sensor, to the drain of the first enhancement MOSFET, used as an output switch, and thus to the positive output signal, so as to effect positive feedback on the operation of the solid-state pulser.
It is also a feature of the present invention through the use of positive feedback, to cause the turn off of the first enhancement MOSFET, which is used as an output switch, to occur quickly and without oscillations.